The Los Angeles American Meteorological Society

Menu:

WHERE DID ALL THE FOG GO?By Jeff Brown

If you were listening to the long range weather forecast a few months ago while most of the southern California coasts, valleys and mountains were blanketed with fog and low clouds most of the day, the forecast was for June Gloom for most of the summer into August. Well, the bummer summer and June Gloom turned into a hot summer and bummer forecast. Just as the forecast for more cloudy days seems to be persistent by late July, the following week turned hot and humid and clear for most areas. Is the long term forecast still for more fog and June like gloom? Or is the long term forecast of low clouds and fog a burn just like the summer heat? According to June Gloom forecasters at JPL and Cal State Los Angeles, the answer is "we're humiliated, but sticking with our original forecast:more fog days and drier than normal weather for the next 10-20 years." "Why are we sticking with our forecast, the PDO says it all". The forecast is for more fog days on average and below normal rainfall on average. Despite the bummer forecast and July and August being among the warmest months of the year, we are still likely to see more fog days especially during the fall and winter months. Looking at the 100 year time series of the PDO, a lot of the changes in the west coast weather in the past several decades can be explained by the PDO.

The PDO or Pacific Decadal Oscillation is like a very large scale El Nino and La Nina in the north Pacific. It essentially tells us that the sea surface temperature pattern across the north Pacific ocean basin changes over a period of decades and this can dictate how big El Nino and La Nina events can be. The PDO appears to be the controlling mechanism of El Nino and La Nina and how intense they will be. This can also tells us how cool and how much rain the California coast can get. As the phrase goes:the past is the key to the future-usually. The warm phase of the PDO is when north Pacific ocean temperatures are cooler than normal and the east Pacific becomes warmer (A) while the western Pacific cools. The cool phase of the PDO is when the western Pacific warms, the north Pacific gets warmer and the east Pacific cools (B). The diagram below shows this.

A BIn this figure, A the blue represents cooler than normal sea surface temperatures (sst's). Yellows and reds indicate warmer than normal sst's. The left diagram A is associated with the positive phase of the PDO which favors El Nino development every 3-7 years. The right diagram shows conditions under the negative phase of the PDO which favors more La Nina type oceanic events

The index below shows the fluctuations in sst's over the past 100 years. The red spikes indicate the positive phase of the PDO. The blue spikes indicate the negative phase.

By looking at this time series, these positive and negative phases last between 10-20 years. We just ended a long -term warm or positive phase (1977-1999) associated with the frequent El Nino's of the 1980's. We are now entering a negative -cool- phase associated with La Nina. We all know that El Nino's are associated with above normal rainfall on average especially with large warm events such as the 1982-83 and 1997-98 events. With La Nina events, we typically see drier years on average according to past events. As we enter the 10-20 year cool phase of the PDO, this is where we can expect to see below average rainfall and cooler temperatures along the coast. Dense fog along coastal cities will also likely be on the increase on average. This does not mean we will not experience heat waves or strong winter storms to affect the state this coming season or next years fall and winter season. On average the winters will be drier and coasts foggier if we indeed switch back to the negative PDO as it appears. The graph below shows the number of fog days at downtown Los Angeles and Los Angeles International Airport from 1950-2000. This graph shows how the number of fog days has fluctuated in accord with the phases of the PDO. As the graph shows we appear to in store for more foggy weather in the years ahead. Aside from this summers heat, we can expect to see more cloudy and gloomy days along the coast as the summer heat turns to fall and winter fog.

The above graph shows during type 1 El Nino years the number of fog days decreases in general. This is due to the warmer sst's off the southern California coast during major El Nino events. LA Downtown temperatures are often warmer than normal due to increase in sst's (from LaDochy, et.al. 2003)

While most climate scenarios are based on computer models of the atmosphere with regards to carbon dioxide emissions and other greenhouse gas chemicals expected to increase over the next several decades, the forecast based on the PDO Index is a natural cycle in the oceanic and atmospheric system and has been going on for the past century as indicated by the index. So the long range 10-20 year forecast is still for more dense fog days.

Powered by Create your own unique website with customizable templates.